MicroRNAs (miRNAs) are noncoding RNAs that bind to target mRNAs and reduce their expression through translational repression or mRNA degradation. Measurements made in myocardial tissue have suggested the miRNAs play a regulatory role in myocardial growth, fibrosis, and remodeling.
MicroRNAs have been isolated from C. elegans, Drosophila, and humans (Lagos-Quintana et al., 2001; Lau et al., 2001; Lee and Ambros, 2001). Several hundred miRNAs have been identified in plants and animals—including humans—which do not appear to have endogenous siRNAs. Thus, while similar to siRNAs, miRNAs are nonetheless distinct. miRNAs thus far observed are approximately 21-22 nucleotides in length and they arise from longer precursors, which are transcribed from non-protein-encoding genes. See review of Carrington et al. (2003). The precursors form structures that fold back on each other in self-complementary regions; they are then processed by the nuclease Dicer in animals or DCL1 in plants. miRNA molecules interrupt translation through imprecise base-pairing with their targets.
Most miRNAs are involved in gene regulation. Some of these miRNAs, including lin-4 and let-7, inhibit protein synthesis by binding to partially complementary 3′ untranslated regions (3′ UTRs) of target mRNAs. Others, including the Scarecrow miRNA found in plants, function like siRNA and bind to perfectly complementary mRNA sequences to destroy the target transcript (Grishok et al., 2001). Some miRNAs, such as lin-4, let-7, mir-14, mir-23, and bantam, have been shown to play critical roles in cell differentiation and tissue development (Ambros, 2003; Xu et al., 2003). Others are believed to have similarly important roles because of their differential spatial and temporal expression patterns.